Spectral sensitization is a well known and important means for extending the spectral sensitivity of a silver halide photographic emulsion from the inherent sensitivity region of silver halide to a longer wavelength region. It is now possible to control the light-sensitive wavelength region almost at will up to the infrared region, depending on the particular purpose, by appropriately choosing the type or structure of a sensitizing dye. However, there still remain some problems to be overcome. Sensitizing dyes for use in spectral sensitization are required to meet, as well as the original requirements that (1) spectral sensitivity distribution is suitable and (2) high sensitivity can be imparted within the desired spectral sensitization region, the following requirements: (3) they do not cause the formation of fog, (4) they do not cause a change in sensitivity or an increase in fog during the production or storage of the light-sensitive material, (5) they do not stain the light-sensitive material by remaining as is or forming colored substances through decomposition after development.
In spectral sensitization within the region of from blue to green, a number of sensitizing dyes are known to be useful, e.g., monomethinecyanines, apomerocyanines, dimethylmerocyanines, pseudocyanines, carbocyanines, hemicyanines, etc. Photographic systems in which the above sensitizing dyes can be used include a facsimile system to send a draft of newspaper to a distant place, a high speed photographic-composing system to prepare a printing form very rapidly, a cathode ray tube display system in which an output from an electronic calculator is displayed and recorded as a figure or character by the use of a cathode ray tube, a microfilm system for duplication, and a lith film system using a light-sensitive material for photomechanical use. In addition, they are used in various silver halide light-sensitive materials such as a microfilm for duplication, a lith film as a light-sensitive material for photo-mechanical use, a light-sensitive material for X-ray use, a multilayer color light-sensitive material, a black-and-white light-sensitive material for lens through camera, and a black-and-white photographic paper. In connection with spectral sensitization, which is essential in preparation of the silver halide light-sensitive materials as described above, various improvements have been described, for example, in Japanese Patent Application (OPI) Nos. 106423/76, 73137/73, 106426/76, 139323/76, 140622/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), Japanese Patent Publication Nos. 12656/74 and 13021/74.
Various types of light sources which are used in the above-described systems include a xenon flash light, a flying spot of a fluorescent substance of a cathode ray tube, a laser light, a xenon arc light, a high pressure mercury lamp, a luminescent semiconductor diode, a glow discharge tube, a tungsten light, and a sunlight. Even in the same system, a single light source is not always used; in some cases, a plurality of light sources are used. Thus, even in the case of a light-sensitive material to be used in a single system, it is required to exhibit high sensitivity to all light sources. In the cathode ray tube display system, for example, many fluorescent substances, such as P-1, P-11, P-15, P-16, P-22, P-22D, P-24, P-31 (which each shows a code number of the cathode ray tube display, for example, P-11 shows a cathode ray tube display comprising a fluorescent substance which has an emission peak of 470 nm generally used for photography), are used and their emission intensity distributions are different. In the photocomposing system, as well as a cathode ray tube display, a xenon flash light and a tungsten light, for example, are used. Furthermore, as well as they have different emission intensity distributions, their emission energy intensities are greatly different. Thus, it is required for silver halide photographic light-sensitive materials for use in the above photographic systems to exhibit high sensitivity to all light sources which might be used therewith.
Although it is very difficult to meet even the above requirement alone, the light-sensitive material should satisfy the above-described requirements (1) to (5) to the greatest extent possible. That is, only a light-sensitive material meeting all the above requirements is commercially usable. In particular, in a reflection type light-sensitive material, the stain or fog is conspicuous compared with a transmission type light-sensitive material. In the case of the reflection type light-sensitive material, even an improvement in whiteness alone has not been disclosed. With the spread of rapid developing systems using an automatic developing machine, stain has recently become a conspicuous problem, although it has not been formed in the conventional technique. This problem cannot be explained as being simply because of the sensitizing dye remaining due to a reduction in the processing time; rather, it is ascribable to a reduction in the drying time and hardening of coated layers of emulsions, for example, so that the light-sensitive material can be passed at high speeds in the automatic developing machine, and by this reason the sensitizing dye may more readily remain in the light-sensitive material after development. If the sensitizing dye remains, not only is the stain increased, but also the development is inhibited. Thus, it is required for the light-sensitive material to have higher sensitivity. Under these circumstances, it has been desired to develop a spectral sensitization technique which permits the production of improved silver halide photographic light-sensitive materials.